AZ/OC are right...
Cashslick/Shaved... are wrong.
I'm not going to bother with much more than that...
If you really want to know A/F make your own wideband O2 sensor... it'd cost like 150-200 bucks...
I wanted to make a couple of them a while back but didn't have the money...
here's a link if anyone is interested... (you have to solder a circuit board so it's not for everyone... I think they have a breadboard version that should be easier for those without real good soldering skills.)...
http://www.diy-wb.com/